Two- or three-dimensional image data is often generated with the aid of state-of-the-art imaging methods and such data can be used for visualizing an imaged examination object and in addition also for further applications.
The imaging methods are frequently based on the detection of X-ray radiation, with data referred to as projection measurement data being generated in the process. Projection measurement data can be acquired with the aid of a computed tomography system (CT system), for example. In CT systems, a combination consisting of X-ray source and oppositely positioned X-ray detector is arranged on a gantry and typically rotates around a measurement space in which the examination object (referred to hereinafter without loss of generality as the patient) is situated. In this case the center of rotation (also known as the “isocenter”) coincides with an axis referred to as system axis z. In the course of one or more revolutions the patient is irradiated with X-ray radiation of the X-ray source, projection measurement data or X-ray projection data being acquired with the aid of the oppositely disposed X-ray detector.
The generated projection measurement data is dependent in particular on the design of the X-ray detector. X-ray detectors typically have a plurality of detection units which in most cases are arranged in the form of a regular pixel array. Each of the detection units generates a detection signal for X-ray radiation incident on the detection units, which detection signal is analyzed with regard to intensity and spectral distribution of the X-ray radiation at specific time instants in order to obtain inferences in relation to the examination object and to generate projection measurement data.
In this case each specific time instant is assigned a specific angle of rotation of the X-ray source around the isocenter, also known as the projection angle. The projection measurement data associated with a specific projection angle at a specific time instant or specific time interval is typically referred to as a so-called “projection”, and the time interval or time instant assigned to a specific projection angle is referred to hereinbelow as the “frame time”.
The projection measurement data is acquired with the aid of CT systems commonly in use today and is used for the purpose of reconstructing a set of sectional images (slices) or, as the case may be, a volumetric dataset. Several hundred to several thousand projections are used for each individual sectional image. The time to acquire a projection (also: a “frame”) typically amounts to a few hundred microseconds (usually between 100 μs and 1 ms). The frame time is generally determined by the gantry's rotational speed and the rotation angle range in which projection measurement data is to be generated for the volumetric dataset.
In the case of what are termed quanta-counting or photon-counting X-ray detectors, the detection signal for X-ray radiation is analyzed with regard to the intensity and the spectral distribution of the X-ray radiation in the form of count rates. The count rates are made available as output data of what is termed a detector channel which is assigned to one detection unit in each case. With quanta- or photon-counting detectors having a plurality of energy thresholds, each detector channel normally generates a set of count rates per projection on the basis of the respective detection signal of the detection unit. In such cases the set of count rates can include count rates for a plurality of different, in particular simultaneously checked energy threshold values. The energy threshold values and the number of energy thresholds to which an energy threshold value is assigned in each case are in most cases predefined as signal analysis parameters for acquisition of the projection.
In this case an upper limit for the number of simultaneously checked energy thresholds of a detector channel is given in that separate signal comparators and counter elements should be present in each case for each checked energy threshold in the detector channel. For cost and space reasons the number of energy thresholds can therefore not be increased ad infinitum, and consequently compromises in terms of the number of analyzed energy thresholds are unavoidable. It is clear in this case that the signal analysis parameters ultimately determine the quality of the analysis of the X-ray radiation and hence of the generated projection measurement data. The quality of the generated projection measurement data is in this case influenced inter alia by the separation of two X-ray radiation quanta in the detection signal which is possible in a temporal interval referred to as “single pulse separation”. Furthermore, the quality of the projection measurement data can also be affected by the energetic interval in which the separation of two X-ray radiation quanta (which is generally represented as voltage distance in the signal) is possible.
All in all, however, it is difficult to specify these signal analysis parameters in a suitable manner in advance so that high-quality projection measurement data can be generated. The present invention provides a remedy in this regard.